Conventionally, as a motor driving circuit for use on an electric vehicle or the like, one having a function of detecting insulation resistance has been proposed. With such a motor driving circuit, for example, on detection of dielectric breakdown, appropriate measures can be taken so that the resulting trouble may be avoided as much as possible. Patent Document 1 identified below discloses an electric automobile system in which, while a controller monitors the voltages of a high-voltage battery and an auxiliary battery, a voltage converter and an electromechanical system are operated in coordination to achieve necessary control. Patent Document 2 identified below discloses a device comprising means for detecting an induced voltage ascribable to an electric leak so that, on detection of an induced voltage, the supply of power may be shut off.